Method for quantitation of collagen in tissue

ABSTRACT

For successful wound healing to occur, the newly formed skin must generate tensile strength through collagen deposition. Measurement of collagen in the granulating wound bed may be predictive of successful wound healing. Existing methods for collagen measurement either require specialized equipment, or do not allow for discrimination of potential interfering molecules. Herein described is an accurate, specific and reliable method to extract and quantify collagen from tissue utilizing high pressure liquid chromatography techniques. The method is sensitive enough to measure the small amounts of collagen found in newly healing wounds.

This application is a continuation of U.S. patent application Ser. No.11/858,737 filed Sep. 20, 2007, now U.S. Pat. No. 7,491,541 issued onFeb. 17, 2009, which claims priority to U.S. Provisional PatentApplication No. 60/846,250, filed Sep. 21, 2006. The entire contents ofthese documents are specifically incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to measuring collagen content intissues. More particularly, it concerns the measurement of collagencontent using HPLC technology. In specific embodiments, the inventionconcerns measuring collagen content in healing wounds.

2. Description of Related Art

Collagen protein is made of polypeptide chains composed of a repeatedsequence of amino acids primarily consisting of hydroxyproline (Hyp),glycine (Gly), and proline (Pro). Collagen is one of the mostpredominant proteins found in the human body, comprising about 80-85% ofthe extracellular matrix (ECM) in the dermal layer of normal(non-wounded) skin tissue. Evidence suggests that collagen's uniquecharacteristics provide normal skin tissue with tensile strength,integrity, and structure. G. S. Schultz, et al., Extracellular matrix:review of its roles in acute and chronic wounds, World Wide Wounds,(August 2005) on the world wide web atworldwidewounds.com/2005/august/Schultz/Extrace-Matric-Acute-Chronic-Wounds.html.Since generation of tensile strength is an important component ofsuccessful wound healing and because collagen is the main proteininvolved in the generation of tensile strength, accurate collagenmeasurements may be predictive of successful wound healing.

Various analytical methods commonly used in biochemistry and analyticalchemistry may be appropriate for quantifying collagen, such asspectrophotometry and gas chromatography. H. Inoue, et al., J. ofChromatography B, 724:221-230 (1999). Most currently availablespectrophotometer calorimetric methods for measuring hydroxyproline aremodifications of the 1950 Neuman and Logan or Stegemann methods, whichmay be sensitive and accurate, but are also cumbersome and problematic.H. Stegemann and K. Stalder, Clinica Chemical Acta, 18:267-273 (1967);B. R. Switzer and G. K. Summer, Analytical Biochemistry 39:487-491(1971).

Another common analytical method is high pressure liquid chromatography(HPLC). HPLC operates by forcing a sample through a specially packedcolumn by pumping a liquid, to which the sample is added, through thecolumn at high pressure. The material packed inside the column isreferred to as the stationary or adsorbent phase, and is usually finelyground powders or gels. The liquid, which is referred to as the mobilephase, is commonly an organic and/or buffered solution. Substanceswithin the sample that are separated during chromatography for study arecommonly called analytes. A chromatogram, the visual output of achromatograph, displays different peaks or patterns corresponding todifferent components (such as analytes) of the separated mixture. HPLCgenerated data allows an analyst to determine if there are interferingor co-eluting peaks, and exactly where these peaks originate. The peaksgenerated by HPLC may be analyzed for separation, whereas it is notpossible to determine if results from a spectrophotometer are due tocontaminating compounds.

Known methods of using HPLC to measure collagen require specializedequipment such as fluorescence detectors or highly specialized columns,which can increase the cost and/or complexity of analysis, potentiallyrendering the procedure prohibitive. H. Inoue, et al; J. ofChromatography B, 757:369-373 (2001); D. A. Martinez et al., DiabetesRes. and Clinical Practice, 591-9 (2003); F. A. Vázquez-Ortíz, et al., Jof Liquid Chromatography & Related Tech., 27, 17 2771-2780 (2004).Inoue, et al. describe a method of using HPLC to determine human serumlevels of prolyl dipeptides, proline, and hydroxyproline as an indicatorof diseases involving collagen metabolism. This method requires tandemHPLC columns and monitoring at two different emission wavelengths.Martinez et al. analyzed hydroxyproline utilizing a reverse phase HPLCmethod with the Waters Pico-Tag® column, which is a dedicated columnspecially packed for amino acid analysis. By quantifying hydroxyprolineand hydroxylysylpyridinoline cross links, Martinez et al. an index ofcollagen content in pig left ventricle was obtained, but collagencontent was not quantified. Vázquez-Ortíz, et al. described a method ofdetermining collagen concentration in meat products such as bologna.This method involves measuring hydroxyproline content using reversephase HPLC and a fluorichrom detector.

SUMMARY OF THE INVENTION

In one embodiment, the invention provides a method of measuring totalcollagen in a tissue comprising: (a) obtaining a tissue sample; (b)processing the tissue sample; (c) separating at least one of glycine,proline, and hydroxyproline in the processed tissue sample byhigh-pressure liquid chromatography to obtain analyte peak(s) area(s);(d) determining concentrations of at least one of the glycine, proline,and hydroxyproline in the tissue; and (e) correlating the concentrationsof at least one of the glycine, proline, and hydroxyproline in thetissue with an amount of total collagen in the tissue.

In another embodiment, the invention provides a method of measuringtotal collagen in a tissue comprising: (a) obtaining a tissue sample;(b) processing the tissue sample; (c) separating glycine, proline, andhydroxyproline in the processed tissue sample by high-pressure liquidchromatography to obtain analyte peaks areas; (d) determiningconcentrations of the glycine, proline, and hydroxyproline in thetissue; and (e) correlating the concentrations of the glycine, proline,and hydroxyproline in the tissue with an amount of total collagen in thetissue.

In certain embodiments, the tissue is from a wound site. In oneembodiment of the invention, the tissue is granulated.

In further embodiments, the invention provides a method of assessing ormonitoring wound healing progress comprising: (a) obtaining a tissuesample from a wound site of a subject; (b) processing the tissue sample;(c) separating at least one of glycine, proline, and hydroxyproline inthe processed tissue sample by high-pressure liquid chromatography toobtain analyte peaks areas; (d) determining concentrations of at leastone of the glycine, proline, and hydroxyproline in the tissue; (e)correlating the concentrations of at least one of the glycine, proline,and hydroxyproline in the tissue with an amount of total collagen in thetissue; and (f) comparing the amount of total collagen in the tissue atthe wound site with the amount of collagen in non-wounded tissue toassess wound healing progress.

In another embodiment of the invention, provided is a method ofassessing wound healing progress comprising: (a) obtaining a tissuesample from a wound site of a subject; (b) processing the tissue sample;(c) separating glycine, proline, and hydroxyproline in the processedtissue sample by high-pressure liquid chromatography to obtain analytepeaks areas; (d) determining concentrations of the glycine, proline, andhydroxyproline in the tissue; (e) correlating the concentrations of theglycine, proline, and hydroxyproline in the tissue with an amount oftotal collagen in the tissue; and (f) comparing the amount of totalcollagen in the tissue at the wound site with the amount of collagen innon-wounded tissue to assess wound healing progress. In certainembodiments, the method is repeated at least once, and the tissue sampleobtained from the same wound site of the same subject for eachrepetition. Repetition may be repeated at least once during a delimitedtime period. In certain embodiments, the time period may be daily,weekly, bimonthly, monthly, quarterly, biannually, annually, two years,three years, four years, or five years. In certain embodiments, themethod may be repeated every other day, every third day, every fourthday, every fifth day, every sixth day, every week, every other week,every third week, every fourth week, every fifth week, or every sixthweek. Other embodiments include comparing the amount of total collagenin the tissue at a first repetition with the amount of total collagen inthe tissue at one or more subsequent repetitions. Comparing the amountof total collagen in tissue at a wound site allows for the monitoringand/or evaluating of the progress of wound healing.

The tissue sample may be obtained from human sources or a variety ofanimal sources such as mammals, amphibians, reptiles, fish, and birds.Some non-limiting mammalian examples include mice, rats, pigs, rabbits,dogs, cattle, and humans. Tissue may be removed from the source byvarious means, for example, cutting, scraping, ablating, abrading, orincising. Removal may be assisted by the use of a tool, for example, aknife, scalpel, razor, or blade.

The invention may be applied to any tissue type that contains collagen.Collagen is a major component of the extracellular matrix that supportsmost tissues. In addition, collagen may be found inside certain cells.Collagen is a main component in soft tissues such as skin, fascia,cartilage, ligaments, and tendons. Collagen is also found in bone andteeth. In certain embodiments of the invention, the tissue is skin. Skinis composed of three primary layers: the epidermis, which is theoutermost layer; the dermis, which has a high concentration of nerveendings and connective tissue; and the hypodermis, or subcutaneousadipose (fat) layer.

There are many different types of collagen found in humans and animal.Some sources have reported as many as 28 different known types.Approximately 80-90% of the collagen found in the human body is acombination of type I (found in skin, tendon, bone, ligaments, dentin,and interstitial tissue), type II (found in cartilage and vitreoushumor) and type III (found in cell culture and fetal tissue). Theinvention may be applied to any type or combination of types ofcollagen.

As used herein a “processed tissue sample” refers to a tissue samplethat has been processed to a state suitable for separating itscomponents by HPLC. Methods for processing tissue for analysis,including HPLC analysis, are known in the art. The methods of theinvention may include processing the tissue sample such that inparticular embodiments processing includes one or more of the following:(a) fat removal from the tissue sample; (b) dehydration of the tissuesample; (c) hydrolyzing the sample to amino acids; and/or (d)derivatization of the sample. All of these processes involve proceduresthat are known and commonly conducted in the art. Fat removal may beconducted in a variety of ways, including physical and chemical. Incertain embodiments, chemical defatting involves washing with solventsor detergents. Certain non-limiting examples of such solvents includeethyl alcohol, acetonitrile, hexane, pentane and/or acetone. Dehydrationmay be accomplished by a variety of processes such as freeze drying(lyophilization), desiccation, a controlled application of heat, and/orapplication of a vacuum. In a specific, non-limiting embodiment, thesample is hydrolyzed by exposure to hydrochloric acid and heat, andderivatized in a solution of 4-dimethylaminoazobenzene 4′-sulfonylchloride in acetone.

In one embodiment, determining the concentrations of the glycine,proline, and hydroxyproline in the tissue comprises: (a) obtainingsamples with known concentrations of glycine, proline, andhydroxyproline; (b) separating the glycine, proline, and hydroxyprolinein the samples by high-pressure liquid chromatography; (c) plotting theknown concentrations of glycine, proline, and hydroxyproline on a graphx-axis by analyte peak areas on the graph y-axis to devise a linearstandard curve; and (d) calculating the concentrations of each of theglycine, proline, and hydroxyproline in the processed tissue sample. Theconcentrations of each of the glycine, proline, and hydroxyproline maybe calculated using a formula:

${{Amino}\mspace{14mu}{Acid}\mspace{14mu}{Conc}\mspace{14mu}\left( {µ\; g\text{/}{mL}} \right)} = \frac{{{Analyte}\mspace{14mu}{Peak}\mspace{14mu}{Area}} - b}{m}$wherein b is the y-intercept and m is the slope of the linear standardcurve; and (e) calculating the concentrations of each of the glycine,proline, and hydroxyproline in the tissue using a formula:

${{Amino}\mspace{14mu}{Acid}\mspace{14mu}{Conc}\mspace{14mu}\left( {{µg}\text{/}{mg}} \right)} = \frac{\left( \frac{{Amino}\mspace{14mu}{Acid}\mspace{14mu}{Conc}\mspace{14mu}\left( {{µg}\text{/}{mL}} \right)}{{dilution}\mspace{14mu}{factor}} \right) \times {final}\mspace{14mu}{sample}\mspace{14mu}{vol}\mspace{14mu}({mL})}{{Tissue}\mspace{14mu}{Wt}\mspace{14mu}{({mg}).}}$

In another embodiment, correlating the concentrations of the glycine,proline, and hydroxyproline in the tissue with the amount of totalcollagen in the tissue comprises using a formula:

${{Total}\mspace{14mu}{Collagen}} = \frac{\begin{matrix}\left( {{\sum\mspace{14mu}{{hydroxyproline}\mspace{14mu}\left( {{µg}\text{/}{mg}} \right)}},} \right. \\\left. {{{glycine}\mspace{14mu}\left( {{µg}\text{/}{mg}} \right)},{{proline}\mspace{14mu}\left( {{µg}\text{/}{mg}} \right)\mspace{14mu}{in}\mspace{14mu}{tissue}}} \right)\end{matrix}}{C}$wherein C is the sum of the percentage of glycine, proline, andhydroxyproline in collagen. The value of “C” has been reported in theart, and may vary with collagen type. In a non-limiting example, “C” is0.55.

Methods of devising a standard curve are well known in the art and astandard curve can be devised by hand or with the assistance of aninstrument or computer. Curve-fitting software programs for computers,calculators, and other instruments may be used to devise a standardcurve.

In one embodiment of the invention, the high-pressure liquidchromatography is reversed phase high-pressure liquid chromatography. Inanother embodiment, the high-pressure liquid chromatography is performedon a C18 column, and in yet another embodiment of the invention, thehigh-pressure liquid chromatography is performed at a pH range of about1 to about 12. Elution of the analytes (glycine, proline, andhydroxyproline) during the high-pressure liquid chromatography may bemeasured with, for example, a photodiode array detector. In oneembodiment, the elution is measured at 436 nanometers. In one embodimentof the invention, the elution of the analytes (glycine, proline, andhydroxyproline) during the high-pressure liquid chromatography are notmeasured with a fluorescence detector.

In yet another embodiment of the invention, the high-pressure liquidchromatography has a mobile phase of about 70% 25 mM potassiumphosphate, pH 11.0 buffer, and about 30% acetonitrile. The mobile phasemay consist of 50%, 60%, 70%, 80%, or 90% buffer. Non-limiting examplesof buffers are potassium phosphate, citric phosphate, Tris, HEPES, MES,or MOPS. The buffer phase may contain 10%, 20%, 30%, 40%, or 50%acetonitrile.

The methods of the present invention are well suited to detecting lowlevels of collagen such as may be found in the early stages of woundhealing. In certain aspects of the invention, the method is used todetect hydroxyproline present in a tissue sample at a concentration thatis 1.0 μg/ml or less. In some aspects of the invention, the method isused to detect hydroxyproline present in a tissue sample at aconcentration that is less than 0.9 μg/ml, less than 0.8 μg/ml, lessthan 0.7 μg/ml, less than 0.6 μg/ml, less than 0.5 μg/ml, less than 0.4μg/ml, less than 0.3 μg/ml, or less than 0.2 μg/ml. In some embodiments,the method is used to detect hydroxyproline present in a tissue sampleat a concentration between 0.12 to 1.0 μg/ml, between 0.12 to 0.9 μg/ml,between 0.12 to 0.8 μg/ml, between 0.12 to 0.7 μg/ml, between 0.12 to0.6 μg/ml, between 0.12 to 0.5 μg/ml, between 0.12 to 0.4 μg/ml, between0.12 to 0.3 μg/ml, or between 0.12 to 0.2 μg/ml. In certain aspects ofthe invention, the method is used to detect glycine present in a tissuesample at a concentration that is 1.0 μg/ml or less. In some aspects ofthe invention, the method is used to detect glycine present in a tissuesample at a concentration that is less than 0.9 μg/ml, less than 0.8μg/ml, less than 0.7 μg/ml, less than 0.6 μg/ml, less than 0.5 μg/ml,less than 0.4 μg/ml, less than 0.3 μg/ml, less than 0.2 μg/ml, or lessthan 0.1 μg/ml. In some embodiments, the method is used to detectglycine present in a tissue sample at a concentration between 0.06 to1.0 μg/ml, between 0.06 to 0.9 μg/ml, between 0.06 to 0.8 μg/ml, between0.06 to 0.7 μg/ml, between 0.06 to 0.6 μg/ml, between 0.06 to 0.5 μg/ml,between 0.06 to 0.4 μg/ml, between 0.06 to 0.3 μg/ml, between 0.06 to0.2 μg/ml, or between 0.06 to 0.1 μg/ml. In certain aspects of theinvention, the method is used to detect proline present in a tissuesample at a concentration that is 1.0 μg/ml or less. In some aspects ofthe invention, the method is used to detect proline present in a tissuesample at a concentration that is less than 0.9 μg/ml, less than 0.8μg/ml, less than 0.7 μg/ml, less than 0.6 μg/ml, less than 0.5 μg/ml,less than 0.4 μg/ml, less than 0.3 μg/ml, or less than 0.2 μg/ml. Insome embodiments, the method is used to detect proline present in atissue sample at a concentration between 0.12 to 1.0 μg/ml, between 0.12to 0.9 μg/ml, between 0.12 to 0.8 μg/ml, between 0.12 to 0.7 μg/ml,between 0.12 to 0.6 μg/ml, between 0.12 to 0.5 μg/ml, between 0.12 to0.4 μg/ml, between 0.12 to 0.3 μg/ml, or between 0.12 to 0.2 μg/ml.

As mentioned above, the methods of the present invention are well suitedto detecting low levels of collagen such as may be found in the earlystages of wound healing. Accordingly, in certain embodiments the presentinvention provides methods of assessing wound healing progress at a timepoint less than 9 days, less than 8 days, less than 7 days, less than 6days, less than 5 days, less than 4 days, less than 3 days, or less than2 days from the creation of the wound. In certain aspects, wound healingprogress is assessed between about 2 to 9 days, 2 to 8 days, 2 to 7days, 2 to 6 days, 2 to 5 days, 2 to 4 days, 2 to 3 days, 3 to 9 days, 3to 8 days, 3 to 7 days, 3 to 6 days, 3 to 5 days, or 3 to 4 days fromthe creation of the wound.

The methods of the present invention are also well suited to detectingcollagen levels over a wide dynamic range. This wide dynamic range isuseful for monitoring collagen levels in wounded tissue throughout thehealing process. In certain aspects, the present invention provides amethod in which the concentrations of each of hydroxyproline, proline,and glycine in a tissue sample are detected over a range of 0.06 μg/mlto 25 μg/ml, 0.12 μg/ml to 25 μg/ml, 0.75 μg/ml to 25 μg/ml, or 0.75μg/ml to 24 μg/ml.

It is contemplated that any method or composition described herein canbe implemented with respect to any other method or composition describedherein.

The use of the term “or” in the claims is used to mean “and/or” unlessexplicitly indicated to refer to alternatives only or the alternativesare mutually exclusive, although the disclosure supports a definitionthat refers to only alternatives and “and/or.”

Throughout this application, the term “about” is used to indicate that avalue includes the standard deviation of error for the device or methodbeing employed to determine the value.

Following long-standing patent law, the words “a” and “an,” when used inconjunction with the word “comprising” in the claims or specification,denotes one or more, unless specifically noted.

Other objects, features and advantages of the present invention willbecome apparent from the following detailed description. It should beunderstood, however, that the detailed description and the specificexamples, while indicating specific embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings form part of the present specification and areincluded to further demonstrate certain aspects of the presentinvention. The invention may be better understood by reference to one ormore of these drawings in combination with the detailed description ofspecific embodiments presented herein.

FIG. 1. Shows a linearity plot for hydroxyproline, glycine, and proline,representative of 1 occasion for injections of amino acid standardmixtures of 0.75 μg/mL-24 μg/mL for hydroxyproline, glycine and proline.Areas for each standard concentration represent values calculated undereach amino acid peak. Correlation coefficients for each amino acid trendline are listed and indicate that all amino acid standard curves arelinear between 0.75 μg/mL-24 μg/mL.

FIG. 2. Shows a typical skin chromatogram from normal, intact porcineskin.

FIG. 3. Shows a spiking study plot for hydroxyproline, glycine, andproline. Porcine tissue sample from normal, intact skin was spiked induplicate with 80%, 100%, and 120% of the 6.0 μg/mL amino acid standardmixture (target standard). The trend line is representative of recoveryfor all three amino acids. The results were so similar that linesoverlap. The correlation coefficients for each amino acid were 0.995(hydroxyproline), 0.988 (glycine), and 0.995 (proline).

DETAILED DESCRIPTION OF THE INVENTION A. The Wound Healing Process

When normal skin integrity is disrupted, for example, as from surgery,successful acute wound healing depends on orderly progression throughfour known phases. These phases are hemostasis, inflammation,proliferation, and remodeling or maturation. During hemostasis and theearly inflammatory phase, vasoconstrictors are released causingcapillaries to constrict, allowing platelets and inflammatory cells tomigrate into the wound bed. During the inflammatory phase neutrophilsare released which help stabilize the wound. R. F. Diegelmann and M. C.Evens, Frontiers in Bioscience, 9:283-289 (2004). Within 2 to 3 daysmacrophages enter the wound. These macrophages are responsible forneutrophil and damaged matrix removal. A. J. Meszaros et al., J. ofImmunology, 165:435-441 (2000). The wound next enters the proliferativephase with the migration of fibroblasts and keratinocytes into thewound. It is these fibroblasts which produce collagen and otherextra-cellular matrix proteins necessary for granulation tissueformation. Granulation tissue is typically perfused and fibrousconnective tissue that grows up from the wound base. During the woundhealing process, collagen molecules form a collagen-fibrin matrix whichfacilitates cell migration into the wound.

During the proliferative phase, the infiltration of fibroblasts iscrucial to the wound healing process. Specific cytokines such asplatelet-derived growth factor (PDGF) and transforming growth factor(TGF-β) are fibroblast regulators, which are involved in the productionof granulation tissue. Furthermore, TGF-β aggressively stimulatesproliferation of fibroblasts, which are the most abundant cell typefound in the wound bed. During the final phase of wound remodeling thedeposition of collagen continues. After two years of remodeling, tensilestrength of a wound will reach a maximum of approximately 80% that ofnormal skin tissue. C. T. Hess and R. S. Kirsner, Advances in Skin &Wound Care, 16, 5:246-257 (2003).

In certain cases, a wound fails to heal in the orderly, predictablestages within the time expected. Such wounds are considered chronic, andsufferers of chronic wounds may have additional emotional and physicalstress due to the failure of the wound to heal. Typically, a chronicwound develops if something causes disruption of the inflammatory phaseor the proliferative phase. Common sources of disruption includeinfection, tissue hypoxia, repeated trauma, the presence of debrisand/or necrotic tissue, and certain diseases such as diabetes. Patientswith chronic wounds are at higher risk for infection, and often report agreat deal of pain. To prevent complications from chronic wounds,certain wounds should be evaluated and monitored. The present inventionprovides accurate, specific and reliable methods for evaluating andmonitoring wounds by quantify collagen from wound tissue. These methodsare sensitive enough to measure the small amounts of collagen found innewly healing wounds.

B. Collagen Structure

Collagen molecules, or “tropocollagen” subunits are rods about 300 nmlong and 1.5 nm in diameter. They are made of three polypeptide strands,each of which is a left-handed helix, which are twisted together into aright-handed coiled coil. Tropocollagen subunits will self-assemblespontaneously, and there is some covalent crosslinking within andbetween the helices. Collagen fibrils are bundled collagen molecules,and collagen fibers are bundles of fibrils. The amino acid arrangementof collagen subunit chains is quite distinctive. The pattern Gly-X-Proor Gly-X-Hyp, where X may be any of various other amino acid residues,is prevalent, and specifically the arrangement Gly-Pro-Hyp occursfrequently.

The present invention provides a method that calculates total collagenby analyzing for these three most abundant amino acids found in collagen(hydroxyproline, glycine, and proline). In one embodiment, totalcollagen is calculated by first calculating the three amino acidconcentrations (μg/mL) based on their respective standard curves.Therefore, the concentration (μg/mg) of hydroxyproline, glycine, andproline in each sample can be determined with the equation

${{Amino}\mspace{14mu}{Acid}\mspace{14mu}{Conc}\mspace{14mu}\left( {{µg}\text{/}{mL}} \right)} = \frac{{{Analyte}\mspace{14mu}{Peak}\mspace{14mu}{Area}} - b}{m}$

where b is the y-intercept and m is the slope, based on the linearcurve. Next, the concentration per sample of wet tissue (μg/mg) wascalculated for each amino acid, by taking the sample weight, dilutionfactor, and final sample volume (mL) into consideration.

${{Amino}\mspace{14mu}{Acid}\mspace{14mu}{Conc}\mspace{14mu}\left( {{µg}\text{/}{mg}} \right)} = \frac{\left( \frac{{Amino}\mspace{14mu}{Acid}\mspace{14mu}{Conc}\mspace{14mu}\left( {{µg}\text{/}{mL}} \right)}{{dilution}\mspace{14mu}{factor}} \right) \times {Sample}\mspace{14mu}{Volume}}{{Sample}\mspace{14mu}{Wt}\mspace{14mu}({mg})}$

Total collagen may then be calculated by taking the concentration(μg/mg) of hydroxyproline, glycine, and proline and calculating a sum ofthe three values and then dividing by the sum of the known percentagesof hydroxyproline, glycine, and proline for the particular type ofcollagen, multiplied by 0.01. This is represented below as “C,” whereinC is the sum of the percent composition of glycine, proline, andhydroxyproline in collagen represented as a fraction. Thus,

${{Total}\mspace{14mu}{Collagen}} = \frac{\begin{matrix}\left( {{\sum{{hydroxyproline}\mspace{14mu}\left( {{µg}\text{/}{mg}} \right)}},{{glycine}\mspace{14mu}\left( {{µg}\text{/}{mg}} \right)},} \right. \\\left. {{proline}\mspace{14mu}\left( {{µg}\text{/}{mg}} \right)\mspace{14mu}{in}\mspace{14mu}{tissue}} \right)\end{matrix}}{C}$

The values for C are available in the literature, and reported valuesmay vary with collagen type. For example, the sum of the knownpercentages of hydroxyproline, glycine, and proline for collagenreported by T. M. Devin, Proteins I: Composition and Structure, in: T.M. Devlin (Ed) Textbook of Biochemistry with Clinical Correlations, JohnWiley & Sons, Inc., New Jersey, 2006, pp. 101, is 55%, and furtherreports that collagen is composed of approximately 9.1% hydroxyproline,33% glycine and 13% proline, for a total of 55% of total collagen.

Collagen must be isolated from a sample removed from a subject and oftenchemically prepared before analytical study. Such procedures may includeseveral steps such as freezing, pulverizing, lyophilizing (dehydrating),hydrolyzation, and derivatization. Other steps may include digestion ofnon-collagen molecules, filtering, precipitation, dialysis, dilution,salvation, or repeated washing. Examples of collagen preparationtechniques known in the art can be found in U.S. Pat. Nos. 5,162,506,4,597,762, and 5,814,328.

C. High Pressure Liquid Chromatography

There are also many specific types of HPLC based upon the material ofthe phases, such as normal, reversed phase, ion exchange, andbioaffinity. For example, reversed phase HPLC consists of a non-polarstationary phase and an aqueous, moderately polar mobile phase, andoperates on the principle of hydrophobic interactions, which result fromrepulsive forces between a polar eluent, the relatively non-polaranalyte, and the non-polar stationary phase. In comparison, ion-exchangechromatography relies upon the attraction between solute ions andcharged sites bound to the stationary phase. Ions of the same charge areexcluded. Several companies make HPLC instruments and accessoriescommercially available, such as Agilent Technologies, Hitachi, andWaters Corporation.

HPLC instruments can be outfitted with different types of detectors, forexample, a photodiode array detector. A photodiode array (PDA) is alinear array of multiple, independent photodiode elements arrangedtogether, for example, on an integrated circuit chip or multiplexer. Forspectroscopy, it is placed at the image plane of a spectrometer to allowa range of wavelengths to be detected simultaneously. Array detectorsare especially useful for recording the full uv-vis absorption spectraof samples that are rapidly passing through a sample flow cell, such asin an HPLC detector.

D. Examples

The following examples are included to demonstrate preferred embodimentsof the invention. It should be appreciated by those of skill in the artthat the techniques disclosed in the examples which follow representtechniques discovered by the inventor to function well in the practiceof the invention, and thus can be considered to constitute preferredmodes for its practice. However, those of skill in the art should, inlight of the present disclosure, appreciate that many changes can bemade in the specific embodiments which are disclosed and still obtain alike or similar result without departing from the spirit and scope ofthe invention.

1. Sample Collection and Preparation

Full thickness, 5 cm diameter excisional wounds were created dorsally on20 domestic swine (Sus scrofa). The skin medallions removed during woundcreation were snap frozen at −80° C. in liquid nitrogen until used forcollagen analysis. These medallions served as samples of normal, intactskin. Wounds were dressed with Duoderm™ (a hydrocolloid dressing) anddressings were changed 3 times per week. On Day 9 post-wounding, theanimals were euthanized and a granulation tissue medallion was removedand frozen at −80° C. until used for collagen analysis. These medallionsserved as samples of initial healing tissues.

After collection, the frozen skin tissue samples were pulverized with amulti sample biopulverizer (Multi sample biopulverizer, Biospec). Next,the samples were lyophilized overnight to remove any excess water.Pulverization breaks up the skin tissue and exposes more surface area,which helps to de-fat the tissue. Following lyophilization, samples weresequentially de-fatted in five, 15-minute washes of 70% ethyl alcohol,100% ethyl alcohol, 100% acetonitrile, and 100% acetone. Samples werethen allowed to dry at room temperature. After drying, the samples werehydrolyzed to amino acids, by addition of 6N hydrochloric acid at 110°C. overnight. Following lyophilization, the samples were dried undervacuum overnight to remove any residual hydrochloric acid and thenreconstituted with water to a final concentration of about 13 mg/mL ofsample.

All specimen results were based on a linear curve ranging from 0.75 to24-μg/mL. A 10-μL sample containing approximately 1.3×10⁻¹ mg/mL of skintissue was derivatized with one milliliter of a 1.3-mg/mL solution of4-Dimethylaminoazobenzene 4′-sulfonyl chloride in acetone. Thederivatized samples were diluted with 500 μL of 50 mM sodiumbicarbonate, pH 9.0, to help neutralize the pH of the sample solution.The derivatized samples were incubated for 10 to 15 minutes at 70° C.and then lyophilized overnight. Following lyophilization, samples werereconstituted with 2 mL of 70% ethyl alcohol and filtered with a Whatman25 mm GD/X PSU filter membrane into an HPLC vial.

2. HPLC Analysis and Validation

All filtered samples were analyzed with a Waters Alliance® 2695 HPLCSystem, equipped with a 2996 photodiode array detector and Empowersoftware for data processing. For the determination of total collagen,hydroxyproline, glycine, and proline were measured at 436 nm. Thisreversed phase HPLC method utilized a preconditioned Phenomenex, Gemini™C18 150×4.6 mm, and 3-micron particle size column. All samples were rununder isocratic conditions with a premixed and prefiltered mobile phaseof 70%, 25 mM potassium phosphate, pH 11.0 buffer and 30% acetonitrile.Each injection lasted 45 minutes with a flow rate of 0.5 mL/minute atroom temperature. Hydroxyproline, glycine, and proline peaks all elutedby 25.0 minutes. The time between 26 and 45 minutes allowed the columnto rinse and equilibrate before the next injection.

The HPLC method was validated for linearity, accuracy, precision,accuracy/repeatability, limit of detection, and limit of quantitation.Linearity was determined by assaying five standards prepared with knownconcentrations of hydroxyproline, glycine, and proline, ranging from0.75 to 24-μg/mL. Each standard concentration was plotted showing areaversus known standard concentration, and the correlation coefficient wascalculated for each. Each linearity used had a correlation coefficientgreater than 0.996. Accuracy was determined by assaying six samples onthe same day and ensuring that the averaged recovery relative standarddeviation (RSD) was less than 15%. On two separate occasions precisionwas assayed by analyzing six injections of a 6.0 μg/mL standard, varyingthe column lots for each occasion and ensuring that the RSD was lessthan 3%. Accuracy/repeatability (robustness) was tested by evaluatingsix different sample preparations, prepared by separate analysts, onthree different occasions and ensuring that the RSD was less than 15%for each occasion and less than 20% for all three occasions averaged.Finally, limit of detection and limit of quantitation was determined byassaying serially diluted standards, diluted below the lowest linearitystandard concentration. Three consecutive injections of each dilutionwere made, and then the chromatography was analyzed for a limit ofdetection/limit of quantitation for hydroxyproline, glycine, andproline. The mean and % RSD were calculated for each serial dilutionconcentration and the peak signal to peak noise ratio was alsocalculated and recorded for each dilution. The limit of detectionconcentration was the concentration which had a signal to noise ratio of3:1. The limit of quantitation was the lowest limit which had a RSD ofless than or equal to 15%.

3. Results

A validation was completed to show that the current method is accurate,specific, and reliable. The method was tested to verify that theexternal standards are linear in a range of 0.75-24-μg/mL forhydroxyproline, glycine, and proline (FIG. 1). The linear correlationcoefficients (r² values) for hydroxyproline, glycine, and proline were0.999±0.001, 0.999±0.002, and 0.999±0.001, respectively. The lowestconcentrations that the method can detect for hydroxyproline, glycine,and proline were found to be 0.12, 0.06, and 0.12-μg/mL, in that order,using a signal to noise ratio of 3:1 to calculate these values. Thelowest concentration that the method can quantify for each standard is0.24 μg/mL for hydroxyproline, 0.12 μg/mL for glycine, and 0.24 μg/mLfor proline. These values were calculated using an averaged peak ratioof 4:1 signal to noise and an averaged percent relative standarddeviation (% RSD) of 7.13.

The accuracy/repeatability testing was an intricate part of this methodqualification and was performed on non-wounded porcine tissue. A typicalskin tissue chromatogram is shown in FIG. 2. The retention times forhydroxyproline, glycine, and proline were 11.3, 18.6, and 24.9 minutes,respectively. Six individual samples were processed and analyzed onthree separate occasions, varying the days and/or the analyst. The meanand relative standard deviation was next calculated per occasion. Theoverall mean and overall relative standard deviation was calculatedusing results from all three occasions. The mean for one occasion was268.27-μg/mg total collagen with a 5.20% RSD. The overall mean (for alloccasions) was 240.82 μg/mg total collagen with a 10.70% overallrelative standard deviation. The accuracy/repeatability results for eachoccasion and overall averages are shown in Table 1. The results indicatethat this method is accurate for analyzing collagen in skin tissuesamples.

TABLE 1 Accuracy/Repeatability result for each occasion and overallaverages. Table shows total collagen measurements for normal, intact,porcine skin for the 6 replicates on 3 separate occasions. TotalCollagen Concentration (μg/mg) Specimen Occasion 1 Occasion 2 Occasion 31 267.370 229.545 206.841 2 268.506 232.795 193.746 3 289.808 212.886210.740 4 257.894 236.700 258.787 5 276.174 244.210 231.339 6 249.883235.559 232.037 Mean 268.273 231.949 222.248 % RSD 5.20 4.55 10.45Overall Mean 240.823 Overall 10.70 % RSD

A spiking study was also included in this method qualification toconfirm method accuracy and to show that this method does not analyzefor bias from the normal, intact skin tissue matrix and that nointerferences exist for the peaks of interest. One intact skin samplewas used for this study and spiked in duplicate with 0%, 80%, 100%, and120% of a 6.0 μg/mL concentration standard containing a mixture of allthree amino acids. The percent recovery was then calculated for each ofthe concentrations with a mean and relative standard deviation.Hydroxyproline recoveries were 101.09% with a 5.93% RSD. Glycinerecoveries were 100.50% with a % RSD of 9.52 and the mean for prolinerecoveries was 101.23% with a % RSD of 5.89. Plots were generated foreach component showing the relationship between the actualconcentrations versus the theoretical concentration. The correlationcoefficient, y-intercept, and slope were calculated for each plot. Thecorrelation coefficients for each plot were 0.995 for hydroxyproline,0.988 for glycine, and 0.995 for proline (FIG. 3). Correlationcoefficients approaching 1 indicate that the actual concentration isequivalent to the theoretical. The results for this spike studyconfirmed that there were no interfering peaks from the skin tissuematrix.

To ensure that the method accurately measures the smaller amounts ofcollagen found in newly healing wounds, porcine granulation tissue from9 day old wounds was analyzed, and the data is shown in Table 2. Allsamples fell within the linear portion of the standard curve. Thus, thetotal collagen concentration in all samples was calculable. The meanamount of collagen found in porcine granulation tissue was 56.846 μg/mgwhereas the mean amount of collagen found in porcine, normal, intactskin was 240.823 μg/mg.

TABLE 2 Quantitation of total collagen from 20 porcine Day 9 granulationtissue samples. Porcine Total Collagen Mean Collagen GranulationConcentration Concentration Tissue Sample (μg/mg) (μg/mg) % RSD 1 39.38856.846 15.36 2 58.405 3 53.981 4 51.483 5 55.015 6 61.028 7 70.000 846.490 9 54.607 10 63.396 11 43.520 12 54.444 13 54.791 14 53.539 1568.339 16 55.649 17 58.321 18 57.327 19 77.103 20 60.093

4. Discussion

This HPLC method required careful optimization due to the close elutiontimes of the hydroxyproline and the derivative peak. A number ofdifferent columns were tested during this process, with best resultsobtained using a Gemini C18 column with a wide pH range. The method wasvalidated based upon USP guidelines found in, Section 501 of the FederalFood, Drug, and Cosmetic Act, <1225> Validation of Compendial Methods,in: (2004) The United States Pharmacopeia 27/The National Formulary 22,United States Pharmacopeial Convention, Inc., Maryland, 2004, pp.2622-2625. Coefficient of correlation values (r²) from the linearitystudies show that all analyte assays were linear between 0.75 and 24μg/mL. Recovery from the spiking study showed that the method isaccurate and no confounding bias was found. Method precision wasdemonstrated by producing precision results of a RSD less than 3% foreach occasion of this study. Accuracy/repeatability of the method wasshown by producing an RSD≦15% on three different occasions and ≦20%overall.

Methods used to quantify the small amounts of collagen produced in thewound at early time points, must be sensitive. The current method wasvalidated using normal, intact porcine skin. Once validated, the methodwas used to quantify collagen levels in porcine granulation tissue toensure that quantitation of collagen in this tissue would fall withinthe linear ranges set for the method. The current method has proved tobe sensitive, accurate and precise between at least a range of 0.75 and24 μg/mL. This method allowed for measurement of the amounts of collagenfound in normal porcine skin (˜241 μg/mg fresh weight) and in a porcine,granulating 9 day old wound (˜57 μg/mg).

All of the compositions and methods disclosed and claimed herein can bemade and executed without undue experimentation in light of the presentdisclosure. While the compositions and methods of this invention havebeen described in terms of preferred embodiments, it will be apparent tothose of skill in the art that variations may be applied to thecompositions and methods and in the steps or in the sequence of steps ofthe methods described herein without departing from the concept, spiritand scope of the invention. More specifically, it will be apparent thatcertain agents which are both chemically and physiologically related maybe substituted for the agents described herein while the same or similarresults would be achieved. All such similar substitutes andmodifications apparent to those skilled in the art are deemed to bewithin the spirit, scope and concept of the invention as defined by theappended claims.

REFERENCES

The following references, to the extent that they provide exemplaryprocedural or other details supplementary to those set forth herein, arespecifically incorporated herein by reference.

-   G. S. Schultz, et al., Extracellular matrix: review of its roles in    acute and chronic wounds, World Wide Wounds, (August 2005)    http://www.worldwidewounds.com    2005/august/Schultz/Extrace-Matric-Acute-Chronic-Wounds.html.-   Inoue, et al., J. of Chromatography B, 724:221-230 (1999).-   H. Stegemann and K. Stalder, Clinica Chemical Acta, 18:267-273    (1967); B. R. Switzer and G. K. Summer, Analytical Biochemistry    39:487-491 (1971).-   H. Inoue, et al; J. of Chromatography B, 757:369-373 (2001);-   D. A. Martinez et al., Diabetes Res. and Clinical Practice, 591-9    (2003);-   F. A. Vázquez-Ortíz, et al., J. of Liquid Chromatography & Related    Tech., 27, 17 2771-2780. (2004).-   R. F. Diegelmann and M. C. Evens, Frontiers in Bioscience, 9:283-289    (2004).-   J. Meszaros et al., J. of Immunology, 165:435-441 (2000).-   C. T. Hess and R. S. Kirsner, Advances in Skin & Wound Care, 16,    5:246-257 (2003).-   U.S. Pat. No. 5,162,506-   U.S. Pat. No. 4,597,762-   U.S. Pat. No. 5,814,328-   Section 501 of the Federal Food, Drug, and Cosmetic Act, <1225>    Validation of Compendial Methods, in: (2004) The United States    Pharmacopeia 27/The National Formulary 22, United States    Pharmacopeial Convention, Inc., Maryland, 2004, pp. 2622-2625.-   T. M. Devin, Proteins I: Composition and Structure, in: T. M. Devlin    (Ed) Textbook of Biochemistry with Clinical Correlations, John Wiley    & Sons, Inc., New Jersey, 2006, pp. 101

1. A method of assessing wound healing progress comprising: (a)obtaining a tissue sample from a wound site on a subject's skin; (b)processing the tissue sample; (c) separating glycine, proline, andhydroxyproline in the processed tissue sample by high-pressure liquidchromatography to obtain analyte peaks areas; (d) determiningconcentrations of the glycine, proline, and hydroxyproline in thetissue; (e) correlating the concentrations of the glycine, proline, andhydroxyproline in the tissue with an amount of total collagen in thetissue; and (f) comparing the amount of total collagen in the tissue atthe wound site with the amount of collagen in non-wounded skin tissue toassess wound healing progress.
 2. The method of claim 1, wherein thetissue is granulated.
 3. The method of claim 1, wherein steps (a)-(f)are repeated at least once, and the tissue sample is obtained from thesame wound site of the same subject for each repetition.
 4. The methodof claim 3, wherein the steps (a)-(f) are repeated at least once duringa time period of between about 3 days and about 2 years.
 5. The methodof claim 3, further comprising comparing the amount of total collagen inthe tissue at a first repetition with the amount of total collagen inthe tissue at one or more subsequent repetitions.
 6. The method of claim1, wherein the subject is a mammal.
 7. The method of claim 6, whereinthe mammal is a human.
 8. The method of claim 1 wherein processingincludes the following steps: (a) fat removal from the tissue sample;(b) dehydration of the tissue sample; (c) hydrolyzing the sample toamino acids; and (d) derivatization of the sample.
 9. The method ofclaim 1, wherein the high-pressure liquid chromatography is performed ona C18 column.
 10. The method of claim 9, wherein the C18 column has a pHrange of about 1 to about
 12. 11. The method of claim 1, wherein elutionof the glycine, proline, and hydroxyproline during the high-pressureliquid chromatography is measured at 436 nm with a photodiode arraydetector.
 12. The method of claim 1, wherein the high-pressure liquidchromatography is reversed phase high-pressure liquid chromatography.13. The method of claim 1, wherein the high-pressure liquidchromatography has a mobile phase of 70% 25 mM potassium phosphate, pH11.0 buffer, and 30% acetonitrile.
 14. The method of claim 1, whereindetermining the concentrations of the glycine, proline, andhydroxyproline in the tissue comprises: (a) obtaining samples with knownconcentrations of glycine, proline, and hydroxyproline; (b) separatingthe glycine, proline, and hydroxyproline in the samples by high pressureliquid chromatography; (c) plotting the known concentrations of glycine,proline, and hydroxyproline on a graph x-axis by analyte peak areas onthe graph y-axis to devise a linear standard curve; and (d) calculatingthe concentrations of each of the glycine, proline, and hydroxyprolinein the processed tissue sample using a formula:${{Amino}\mspace{14mu}{Acid}\mspace{14mu}{Conc}\mspace{14mu}\left( {{µg}\text{/}{mL}} \right)} = \frac{{{Analyte}\mspace{14mu}{Peak}\mspace{14mu}{Area}} - b}{m}$wherein b is the y-intercept and m is the slope of the linear standardcurve; and (e) calculating the concentrations of each of the glycine,proline, and hydroxyproline in the tissue using a formula:${{Amino}\mspace{14mu}{Acid}\mspace{14mu}{Conc}\mspace{14mu}\left( {{µg}\text{/}{mg}} \right)} = {\frac{\left( \frac{{Amino}\mspace{14mu}{Acid}\mspace{14mu}{Conc}\mspace{14mu}\left( {{µg}\text{/}{mL}} \right)}{{dilution}\mspace{14mu}{factor}} \right) \times {final}\mspace{14mu}{sample}\mspace{14mu}{vol}\mspace{14mu}({mL})}{{Tissue}\mspace{14mu}{Wt}\mspace{14mu}({mg})}.}$15. The method of claim 1, wherein correlating the concentrations of theglycine, proline, and hydroxyproline in the tissue with the amount oftotal collagen in the tissue comprises using a formula:${{Total}\mspace{14mu}{Collagen}} = \frac{\begin{matrix}\left( {{\sum{{hydroxyproline}\mspace{14mu}\left( {{µg}\text{/}{mg}} \right)}},{{glycine}\mspace{14mu}\left( {{µg}\text{/}{mg}} \right)},} \right. \\\left. {{proline}\mspace{14mu}\left( {{µg}\text{/}{mg}} \right)\mspace{14mu}{in}\mspace{14mu}{tissue}} \right)\end{matrix}}{C}$ wherein C is the sum of the percent composition ofglycine, proline, and hydroxyproline in collagen divided by
 100. 16. Themethod of claim 15, wherein C equals 0.55.